Mesocyclone

A mesocyclone is a dramatic, threatening sight that few who witness it will ever forget. A huge, circular, flat-bottomed mass of cloud descends from the black base of a violent thunderstorm, its surface whorled and twisted by strong winds. Hanging close to the ground, this vast, ponderous mass can be miles across, and can be seen visibly rotating, churning with inward, powerful forces. Ragged wisps of cloud blow around it, while deep darkness lies beneath it. Large hail may fall from it, or a tornado may reach out to the ground from its underside to uproot trees and shatter houses in its path.

Mesocyclones are cyclonic, or turning, patterns in thunderstorms that often lead to tornadoes, and sometimes spawn dozens of tornadoes in a short amount of time. They are called 'cyclones' because they rotating storm systems, and the 'meso-' part of the name, meaning 'middle,' indicates their size. Since they are one to six miles across, they are larger than the smallest cyclones -- tornadoes -- and smaller than full-scale extratropical cyclones. They are caused by large, long-lasting, rotating updrafts that form in strong thunderstorms.

The sight of a mesocyclone is a warning that very severe weather is about to occur in the area. Baseball-sized hail, extremely strong wind gusts, and strong tornadoes are all possible, in addition to the normal thunderstorm weather of heavy rain and lightning. This is a weather sign that you should always heed, and you should try to get into a strongly-built house or other building if you are outdoors because of the dangerous weather the mesocyclone is almost certain to produce.

The creation of a mesocyclone

Mesocyclones are the product of severe thunderstorms, and though no conclusive research has been done to prove this, they are believed to be caused by the interaction of strong wind shear inside the storm with a powerful thunderstorm updraft.

The mesocyclone starts when strong horizontal wind shear occurs towards the back of the thunderstorm. This causes a large volume of air to turn into a huge, spinning tube. This tube then encounters one of the powerful updrafts of the thunderstorm, and combines with it, tilting upwards while retaining its rotating, tubular shape. When it is fully vertical, it becomes a mesocyclone, and often produces a jutting wall cloud underneath the storm's base. Mesocyclones always occur in the southwest part of the parent thunderstorm.

A mesocyclone can sometimes become taller than the thunderstorm that caused it. In this case, it will make a bulge above the flat anvil of the thunderstorm called a cloud dome.

Mesocyclones are the focus for extremely violent weather. Destructive winds are often found near them, as is hail of baseball size or larger -- which can damage vehicles and injure people badly. The worst damage, however, is caused when mesocyclones spawn tornadoes, which they do about half the time. Tornadoes from mesocyclones are often very violent and destructive, and clusters of them may erupt from the wall cloud base.

A mesocyclone lasts an average of one to three hours before giving out, although some can last for much longer and may even survive the thunderstorm that created them as an air vortex in the troposphere that will spawn more thunderstorms in the next day or two. These vortices are known as mesoscale convective vortices and can even develop into tropical storms if they move out from land over warm tropical waters.

Mesocyclone electromagnetic fields?

A few reliable magnetic field readings have been taken near active mesocyclones, and these measurements seem to show that the outer ring of the mesocyclone has a magnetic field much stronger than the normal magnetic field of the planet. At the same time, the center of the mesocyclone vortex has a very weak magnetic field. This has not been extensively studied yet, but it suggests that mesocyclones might be electromagnetic phenomena as well, and not simple whorls of wind and water.